Benzoxazolin-2-(3H)-one (BOA) has been tested in many plants species, but not in soybean (Glycine max). Thus, a hydroponic experiment was conducted to assess the effects of BOA on soybean photosynthesis. BOA reduced net photosynthetic rate, stomatal conductance, and effective quantum yield of PSII photochemistry without affecting intercellular CO2 concentration or maximal quantum yield of PSII photochemistry. Results revealed that the reduced stomatal conductance restricted entry of CO2 into substomatal spaces, thus limiting CO2 assimilation. No change found in intercellular CO2 concentration and reduced effective quantum yield of PSII photochemistry revealed that CO2 was not efficiently consumed by the plants. Our data indicated that the effects of BOA on soybean photosynthesis occurred due to the reduced stomatal conductance and decreased efficiency of carbon assimilation. The accumulation of BOA in soybean leaves reinforced these findings. and Obsahuje bibliografii